Molla Engineering

Unloading valves are pressure-control devices that are used to dump excess fluid to the tank at little or no pressure. Unloading valves are similar to Pressure Relief Valve. Both types of valves send fluid back to a tank when a preset pressure is reached. However, unlike a Pressure Relief · Valve, which is activated using pressure from its own line. The Unloading Valve dumps  the fluid when the fluid from an external line reaches a certain pressure. A common application is in high-low  pump circuits where two pumps move an actuator  at a high speed and low pressure. The circuit then shifts to a single pump providing a high pressure to perform work ,It is used in application in which the first portion of cylinder stroke requires - High Flow (Speed) and Low Pressure (Force) for the reminder of the cylinder stroke which requires- Low Flow and High Pressure. A pilot-operated  unloading relief valve is the same as a pilot-operated relief valve with the addition of an...

  Unlike in unbalanced vane pump, the balanced vane pump has two intake and two outlet ports which are diametrically opposite to each other. Also a balanced vane pump has an elliptical housing instead of a circular cam ring. This configuration creates two diametrically opposed volumes. The two high pressure zones balance the force on the rotor shaft and hence a complete hydraulic balance is achieved. In this pump, the rotor is mounted off center. At regular intervals around the curved surface of the rotor are rectangular vanes that are free to move in a radial slot. As the rotor revolves, the vanes are thrown outwards by centrifugal force to from a seal against fixed casing. The eccentricity of the revolving rotor produces a partial vacuum at the suction side of the pump, causing an inflow of liquid. This is carried to other side of the pump in the space between the rotor and the fixed casing.

                                                                                                                           Schematic representation In the double acting hydraulic cylinder, liquid pressure can be applied to either side of the piston, thereby providing a hydraulic force in both directions. The double acting cylinders are mostly used in application where large stroke lengths are desired. Construction: There are five basic parts of the double acting cylinders; Base cap with port connections Bearing cap with port connection Cylinder barrel Piston Piston rod The components are designed in such way that the end caps and pistons are common to all cylinders of the same diame...

 The basic principle of this type of pump is that a pair of gears being driven by an external means. The gear motion generates suction at inlet by creating a partial vacuum, which causes fluid to be drawn into the pump housing. The fluid is drawn through the pump and is displaced as the teeth mesh on the outlet side. External gear pump have two mating gear that are turned in a closely fitted casting.Each gear is mounted on a shaft which is supported on bearing in the end covers. In this drive shaft is coupled to the prime mover. Two ports, inlet and outlet are provided directly on the opposite side of the mesh point of the gears. The larger and straight ports are preferred for better performance. The driving shaft is usually connected to the upper gear of the pump. The rotation of the driver gear causes the follower gear to turn. When the pump starts rotating, the rotation of the gear forces air out of the casing and into the discharge pipe. This removal of air from the pump casing...

  Oil from the pump flows through the flow central valve and the flow is restricted. The approach to the adjustable restrictor is also through the pressure compensator spool. There is enough passage and space around the compensator for the oil to reach the restrictor. Let us say the pump flow is 221/min and the restrictor is adjusted to allow 101/min. There is a balance flow of 121/min which will be building up pressure in the pipe line P. The load pressure from port A of the cylinder is felt on the outlet of the flow control valve. If the spring rating is 1 bar and the load pressure at port A is 50 bar then the pressure felt on the left-hand side of the pressure compensator is 51-bar. The pressure on the right side of the compensator slowly builds-up. When this pressure reaches a pressure level just above 51-bar, the pressure compensator moves towards the left compressing the spring. In moving the passage to the restrictor is narrowed by the landing of the pressure compensator. Th...

  The rotor and vanes are contained within a stationary, elliptical cam ring and having two sets of internal ports. One set for inlet and other set for outlet during each revolution. A pumping chamber is formed between any two vanes twice in each revolution•. The two inlets and outlets are 180° apart. The rotor is located close to the wall of the cam, so a crescent shape cavity is formed.                                                                  Balanced Vane Pump As the rotor turns centrifugal force causes the vanes to move outward, such that the outer edge of the vanes stays in touch with the inside surface of the asymmetrically shaped cam ring. The asymmetrical shape of the cam ring causes the vanes to move in and out of the slots as the rotor turns. As the vanes moves past the suction port of the pump a vacuu...

 The 4/3 Direction Control valve has four ports marked as P, A, B, T and three distinct positions. Where P is power port, T is return port and A&B are actuator ports.                                                                  (Spool Position: 1)                                                               (Spool Position: 2) Three position four way (3/4) valves are used in double acting cylinders to perform advance hold and return operation to the piston. These types of valves have three  switching positions. They have a variety of possible flow path configurations but have identical flow path configuration. When the centered path is actuated port A an...

  Cheek valves are the most commonly used and simplest types of direction control valve. The cheek valve is a two way valve. The check valves are used  To allow the free flow in only one direction. To prevent any flow in the other direction Since check valve block the reverse flow of the fluid they are also know as non-return valve.   Hydraulic Circuit Using Check Valve                                                 PCV 1 and PCV 2 - Pilot Operated Check Valve          C - Double Acting Cylinder D - Direction Control Valve T - Tank F - Filter P - Pump R - Relief Valve

 There are four types of gear pump External Gear Pump Internal Gear Pump Screw Pump Lobe Pump The basic principle of external gear pump is that a pair of gears being driven by an external means. The gear motion generates suction at inlet by creating a partial vacuum which causes fluid to be drawn into the pump housing. The fluid is drawn through the pump and is displaced as the teeth mesh on the outlet side. Construction: External gear pump have two mating gear that are turned in a closely fitted casing. Each gear is mounted on a shaft which is supported on bearing in the end covers. In this drive shaft is coupled to the prime mover. Two ports inlet and outlet are provided directly on the opposite side of the mesh point of the gears. The larger and straight ports are preferred for better performance. The driving shaft is usually connected to the upper gear of the pump. The rotation of the driver gear cause the follower gear to turn. Operation: When the pump starts rotating the rota...

 Merits of pneumatic system: The working medium of pneumatic system is air which is easily and freely available. Air is dry and hence any pneumatic system is clean. Air cannot be exploded. The pneumatic equipments such as chipper,grinder,rotary actuators,liner actuators are having comparatively low cost and light in weight. Pneumatic system are not having return lines. Hence the total cost of system is low. Demerits of pneumatic: Pneumatic system can be used only up to 10 bar pressure. If the pressure of stored air in air tank drops below required pressure then pneumatic system stops working. It will start only after required pressure is developed in Air tank. Due to compressing nature of air accurate motions in actuator are not possible to obtain. It cannot be used for precise movement of tool of CNC/VMC.

 Advantages : Ease and accuracy of control multiplication of forces Constant force and torque Simplicity, safety and economy High power to weight ratio Automatic protection against overloads Greater efficiency and economy due to low friction losses Relatively smooth and quite operation Multiplication of forces  Limitation : Leakage poses many problems to the operations as well as operators Fire hazards - Flammable hydraulic fluids may pose fire hazards thus limiting the upper limit of working temperature Special treatment to hydraulic elements to protect against rust corrosion and dust Ageing and chemical deterioration of hydraulic fluid Hydraulic System : Confined pressurized system that using moving liquids like water,oil Hydraulic is generally not clean system due to the risk of oil leaks Hydraulic can smoothly lift and move heavy loads, because the hydraulic oil is not compressible Hydraulic systems are typically used for high force where stiffness in position is necessary...

The controlled movement of part or a controlled application of force is a common requirement in the industries. These operations are performed mainly by using electrical machines or diesel, petrol and steam engines as a prime mover. These prime movers can provide various movement to the objects by using some mechanical attachments like screw jack,lever,rack and pinion etc. However these are not the only prime movers. The enclosed fluids (liquid and gases) can also be used as prime movers to provide controlled motion and force to the objects or substance. The specially designed enclosed fluid system can provide both linear as well as rotary motion. The high magnitude controlled force can also be applied by using these systems. This kind of enclosed fluid based systems using pressurized liquids as transmission media are called as hydraulic system. The hydraulic system works on the principle of Pascal's law which says that the pressure in an enclosed fluid is uniform in all directions...